Update the "This is an alpha release" wording to "this is beta release"

Differential Revision: D64374464

Author: mergennachin

docs/source/index.rst

@@ -4,11 +4,12 @@ Welcome to the ExecuTorch Documentation
 =======================================
 
 .. important::
-   This is an alpha release; the ExecuTorch APIs and the ``.pte`` binary format
-   may change in incompatible ways before stabilizing in a future beta release.
-   When deploying models, we currently recommend using a version of the runtime
-   built from the same git revision that was used to generate the ``.pte`` file.
-   Once the format has stabilized, this will no longer be necessary.
+   This is a beta release. As of this ExecuTorch beta release, the API
+   will follow the [lifecycle and deprecation policy](./api-life-cycle)
+   and ``.pte`` binary format will comply with the *runtime compatibility policy* (TODO: add link).
+   This ensures that application developers can update to the latest version of ExecuTorch
+   without breaking existing integration code, in accordance with these policies.
+   If any issues arise or compatibility breaks occur, please report them.
 
    We welcome any feedback, suggestions, and bug reports from the community
    to help us improve the technology. Please use the `PyTorch Forums